All electrical devices require electrical energy in order to function. Many electrical devices receive electrical energy from a power supply. For example, a power supply may obtain energy from a source, such as an energy transmission system, battery, etc., and provide a voltage and current to the electrical device (which may be referred to as the load) to enable its operation.
Depending on the type of load, it may be desirable to vary the voltage and/or current provided to the load. For example, the intensity and/or color of certain types of light sources, such as a light emitting diode (LED), may be varied depending on the voltage provided by the power supply. In addition, the speed of certain types of motors may be controlled by varying the power supply applied to the motor.
In conventional systems, when only partial power is needed, the output of the power supply is reduced, for example, by a variable resistor network connected in series with the motor. This adjusts the amount of current flowing through the motor, but also wastes power as heat in the resistor elements. Furthermore, there may be inefficiencies realized in the motor itself when operating at less than full power.
Pulse-width modulation (PWM) is a technique used for controlling power to electrical devices, while maintaining a greater efficiency and/or ease of control. The average value of voltage and/or current fed to the load is controlled by turning the power supply on and off at a rapid pace. The PWM switching frequency is typically much faster than what would affect the load and can vary depending on the requirements of the load. The term “duty cycle” describes the proportion of time that the power supply is on (i.e., providing power to the load) to a regular interval or period of time. Thus, a low duty cycle corresponds to low average power, because the power is off for most of the time. The duty cycle may be expressed as a percentage, with 100% being fully on and 0% being fully off.
With PWM, the load is being powered on and off repeatedly at a certain frequency and duty cycle. This causes an emission of radiation, such as electromagnetic interference (EMI) at the switching frequency and, to a lesser extent, at its harmonics. This emission may be substantial in the case of large reactive loads (i.e., loads with a large component of inductance or capacitance, such as motors). This emission may be undesirable for a number of reasons, such as regulatory compliance issues, negative effects on the load (e.g., noise in an audio system), or other reasons.
Conventionally, the effects of emission of radiation may be reduced by using a spread spectrum clock. In spread spectrum clocking, the switching frequency is continuously or periodically varied according to a switching profile (e.g., triangle, “Hershey kiss”, etc.) so that the emissions are spread across a range of frequencies. This may reduce the peak emission at any given frequency, potentially putting the system in compliance with the regulations, or reducing the negative effects of the EMI.
Spread spectrum clocking is effective in application where only the average power is important. Although the average power remains the same over a given number of cycles, the instantaneous power applied to the load may vary significantly. In certain applications, precise timing on a cycle-by-cycle basis may be important. One such application is three-phase motor control. Spread spectrum clocking typically causes the clock frequency (which controls the switching) to either constantly change or change in small discrete steps very frequently. Although the average duty cycle will remain constant, the duty cycle for each PWM period will be varying. This can have negative effects on the timing for sensitive loads.
Consider, for example, an 8-bit PWM circuit configured for a 1/256 duty cycle. For 1 out of every 256 clock periods, the PWM output is high (i.e., the “mark” period), and for the remaining 255 clock periods (i.e., the “space” period), the PWM output is low. Thus, if the clock is spread, for example, with 5% modulation, the mark period may be up to 5% higher or lower than the average clock period. In an extreme example, the mark period may be 5% higher than average and the space period may be 5% lower than average, resulting in a duty cycle error of ˜9.5%.